1. Field of the Invention
The present invention relates to a power amplifier, and more particularly, to a power amplifier with noise shaping function.
2. Description of Related Art
Referring to FIG. 1, a common power amplifier, particularly, an audio power amplifier, adopts a negative feedback design to achieve a stable gain. Meanwhile, a low-pass filter is further connected in series to eliminate the high-frequency noise, wherein the low-pass filter is usually an integrating circuit. As shown in FIG. 1, a feedback circuit 110 converts output signals Vo1 and Vo2 into a single-end mode feedback signal Vsum_P. The feedback signal Vsum_P and the input signal VI pass through the integrator 120 to filter the noise.
However, since the amplifier has evolved from the vacuum tube to the transistor, noise interference has always been the largest problem suffered by the audio power amplifier. The main source of noises is divided into two types, wherein one is power supply hum. The output power amplification stage 130 withdraws a high current from the power supply end VDD, but the power supply end VDD cannot be effectively filtered. Therefore, when the gate signal is triggered, and requires a pure high current to passes through the power supply end VDD, the high current contains noises and is not pure, such that the noises are fed into the whole circuit through the feedback circuit 110. The above content can be obtained with reference to Paragraph 2 of Chapter 8 of “Microelectronic Circuit” (fourth version) written by Adel S. Sedra & Kenneth C. Smith. Therefore, the common solution is to add a filter in the feedback circuit 110 to eliminate noises. However, in such design, as shown in FIG. 1, the effect of eliminating noises by a conventional filter is not desirable.
The second source of noises is radio frequency interference. When a radio frequency signal gets close to the audio amplifier, the circuit with a relatively poor common mode rejection ratio receives the radio frequency together as the noise interference of the audio amplifier itself, for example, the buzz sound produced when a mobile phone gets close to a speaker. However, the conventional audio amplifier does not use a fully differential architecture to increase the common mode rejection ratio.